This invention relates generally to systems and methods for driving a solenoid and, more specifically, to systems and method for driving a solenoid that controls a valve in a mass flow controller (MFC).
Many manufacturing processes require that the introduction rates of process gases into a process chamber be strictly controlled. These types of processes use mass flow controllers (MFCs) to control the flow rate of gases. An MFC may control the gas by implementing a solenoid driver circuit to control a valve. The flow rate into the chamber is proportional to the valve opening. In turn, the valve opening is proportional to a current flowing through a solenoid winding.
A basic circuit for typical solenoid driver is shown in FIG. 1. Solenoid driver 10 includes a voltage source 12, a control element 14 such as a transistor, and a load device, solenoid 16. The actual current through solenoid 16 is given by VL/RL, where VL is the voltage across solenoid 16 as controlled by device 4, and RL is the resistance of solenoid 16. RL may vary as a result of operating temperature in solenoid 16. Solenoid driver 10 is continually controlled by means of changing a voltage across solenoid 16 through the use of control element 14. The impedance of solenoid 16 is both inductive and resistive. Typical inductance values range between 1H-4H, and corresponding resistance values range from 100 xcexa9-300xcexa9. Supply voltage 12 used to drive the current in solenoid driver 10 may be in the range of 24 volts (xc2x112 volts) to 36 volts (xc2x118 volts). The voltage VL applied across solenoid 16 is typically between 10 volts and 18 volts, depending on operating parameters such as the desired valve opening and the pressure drop across the MFC device.
Unfortunately, there are two disadvantages with the typical solenoid driver 10 illustrated in FIG. 1. The first disadvantage is that the force exerted by solenoid 16 is proportional to the current flowing through its windings and only indirectly proportional to the voltage across it. If the solenoid voltage is controlled, an additional time delay is introduced in the feedback loop and this delay may cause stability problems.
A second disadvantage of the circuit shown in FIG. 1 is that power is often wasted in control element 14, especially when the difference between supply voltage 12 and the voltage VL across solenoid 16 is large. Denoting supply voltage 12 as VS and given VS, VL, and RL, the dissipated power in control element 14 is equal to (VS-VL)xc3x97VL/RL. The wasted power is dissipated as heat in control element 14 This dissipation is undesirable for two reasons. First, the dissipated power reduces the overall power budget of the system and may violate a power limit imposed by a customer on the MFC. Also, the heat generated through control element 14 may cause problems due to a lack of forced cooling such as a fan inside the unit.
Therefore, it is desirable for a solenoid driver to dissipate as little heat as possible so that the need for a cooling mechanism is reduced or eliminated. Also, it is desirable to reduce the energy consumption of the control element in a solenoid driver so that the control element minimizes the demands placed on the overall power budget of the system.
The present invention provides a system and method for driving a solenoid that substantially eliminates or reduces disadvantages and problems associated with previously developed systems and methods for driving a solenoid.
More specifically, the present invention provides a system and method for a pulse width modulated (PWM) solenoid driver. The system for driving a solenoid includes a solenoid, a current set-point for establishing a desired current flow through the solenoid, and a step-down regulator circuit for controlling the current through the solenoid based on the difference between the desired current flow and the actual current flow through the solenoid.
The present invention provides an important technical advantage by reducing the amount of power wasted due to dissipation through a control element such as a transistor. The step-down regulator provides a minimal voltage drop due to the low resistance associated with it. Therefore, minimal power is dissipated through the internal resistance of the step-down regulator. This reduces the overall power budget needed to drive the solenoid and consequently reduces the cost associated with implementing the solenoid driver.
Another technical advantage of the present invention is that heat associated with the loss of energy through a control element such as a transistor used in a typical solenoid driver is much reduced. Therefore, the necessity of forced cooling such as a fan inside the unit is eliminated.